On the day of the incident, operators attempted to open a valve on spare isobutane pump in order to put the pump into service. The valve was manually operated by a handwheel, which connected to piece of equipment called a gearbox, that contained gears used to make it easier to open and close the valve. The gearbox, however, was not working properly and the valve would not open. The operators, following accepted practice, then removed the gearbox and its support bracket from the valve.

Unknown to the operators, the valve was one of 3% of the total number of valves in the alkylation unit that were of an older design. This 30-year-old design used bolts to connect the gearbox and its support bracket to the valve and also secured a critical piece of pressure-containing equipment called the top-cap. When removed, the bolts leave the top-cap disconnected and the valve vulnerable to pressure.

With the bolts and gearbox removed, and the top-cap not secured, the operators attempted to open the valve using a pipe wrench, and the valve immediately failed and came apart.

Pressurized isobutane escaped from the valve, forming a flammable white vapor cloud, which likely found an energized welding machine roughly 70 feet away from the initial release, and ignited. The resulting fire seriously injured one ExxonMobil employee and three contractors working in the area.